JP2009165270A - Power conversion circuit and air conditioner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To set backward voltage to an appropriate value for completing reverse recovery in so-called dead time in accordance with current flowing in a load. <P>SOLUTION: A power conversion circuit includes first and second wirings 81 and 82, IGBT 31, MOSFET 41, a DC voltage supply 52 and a switching element 22. A parasitic diode 41a is parasitic in MOSFET 41. The DC voltage supply 52 outputs variable DC voltage. The switching element 22 is arranged between the DC voltage supply 52 and the parasitic diode 41a. Only when IGBT 31 and MOSFET 41 are both non-conductive, a backward voltage is applied to the parasitic diode 41a. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a power conversion circuit, and more particularly to an inverter that employs a MOS field effect transistor as a so-called low arm side switching element.

  For example, an inverter employed for driving a motor of an air conditioner has a higher operation ratio at a light load. Therefore, from the viewpoint of power saving, a reduction in loss at a light load is required.

  In an inverter, a series connection (often referred to as a leg) of a pair of switching elements connected to a pair of DC buses (often referred to as a DC link) is provided in parallel with the same number of phases of the load. It is done. Of the pair of switching elements, the one connected to the high potential side DC bus is commonly referred to as the high arm side switching element, and the one connected to the low potential side DC bus is commonly referred to as the low arm side switching element. .

  In order to reduce the conduction loss of these switching elements at light load, it is desired to reduce the on-resistance. In general, the on-resistance can be designed to be smaller by adopting the MOSFET than adopting the IGBT as the switching element.

  IGBT is an abbreviation for an insulated gate bipolar transistor. MOSFET refers to a MOS field effect transistor. Here, MOS is used in the past for metal / oxide / semiconductor laminated structures, and is thought to have originated from the acronym Metal-Oxide-Semiconductor. However, in view of common technical knowledge, the term “MOS” as used herein is not only an abbreviation derived from the word source, but also broadly includes a conductor / insulator / semiconductor laminated structure.

  However, generally, the smaller the on-resistance of the MOSFET, the worse the reverse recovery characteristics of the parasitic diode parasitic on the MOSFET. As the reverse recovery characteristic is worse, there is a problem that the loss during switching increases.

  The magnitude of the reverse recovery current generally increases with the magnitude of the applied voltage that causes reverse recovery, and also increases with the magnitude of the load current. In the switching element of the inverter, the DC voltage of the DC link becomes an applied voltage that causes reverse recovery. In the general-purpose inverter, the DC voltage of the DC link is about 250V. This voltage value is the voltage across the MOSFET's parasitic diode, and also increases the reverse recovery current, which increases the loss during reverse recovery.

  In Patent Documents 1 to 3 listed below, a voltage is applied to the parasitic diode by a separately prepared DC circuit in a so-called dead time period in which both the high arm side and the low arm side are off. The reverse recovery of the parasitic diode is intended. By setting this applied voltage to be smaller than the DC voltage of the DC link, the reverse recovery current is reduced, and the voltage across the parasitic diode when this flows is reduced, thereby reducing the reverse recovery loss. .

Japanese Patent Laid-Open No. 10-327585 JP 2006-141167 A JP 2006-141168 A

  However, in Patent Document 1, since the voltage applied to the parasitic diode is fixed, reverse recovery may not be completed during the dead time when the load current is large. In addition, when the voltage is large, the loss during reverse recovery cannot be effectively reduced.

  The present invention has been made to solve the above-described problem, and an object thereof is to set a reverse voltage to an appropriate value in order to complete reverse recovery during a so-called dead time according to a current flowing through a load. It is said.

  A first aspect of the power conversion circuit according to the present invention includes a first wiring (81), and a second wiring (82) to which a positive DC voltage (Ed) is applied to the first wiring. The first switching element (31) connected between the first wiring and the load (91), and connected between the second wiring and the load (91), A MOS field-effect transistor (41) having a parasitic diode (41a) that conducts current only in one direction from the wiring to the load; a DC voltage source (52) that outputs a variable DC voltage; and the DC voltage source And applies a reverse voltage to the parasitic diode only when both the first switching element and the MOS field effect transistor are non-conductive. 2 switching elements (2 ) And a.

  A second mode of the power conversion circuit according to the present invention is the first mode, wherein a current (I) flows out from a connection point between the first switching element and the MOS field effect transistor to the load. And a voltage control circuit (51) for controlling the DC voltage of the DC voltage source (52) to a greater extent as the current increases.

  The air conditioner concerning this invention is provided with the power converter circuit of Claim 1 or Claim 2. The load (91) is a coil provided in the motor (9). The air conditioner further includes a compressor (81) that is driven by the motor (9) and compresses the refrigerant.

  According to the first aspect of the power conversion circuit of the present invention, the second switching element is turned on before the first switching element is turned on and the reverse voltage is applied to the parasitic diode by the positive DC voltage. By using a method of applying a small reverse voltage from a DC voltage source, it is possible to reduce power loss during reverse recovery of the parasitic diode. Moreover, since the reverse voltage for reverse recovery to the parasitic diode is variable in this way, the reverse voltage is set to an appropriate value in order to complete reverse recovery in the so-called dead time according to the current flowing through the load. Can be set.

  In order to reduce the reverse recovery loss of the parasitic diode, it is desirable to reduce the reverse voltage for reverse recovery. On the other hand, in order to complete reverse recovery within a predetermined period, it is necessary to increase the reverse voltage for reverse recovery of the parasitic diode as the current flowing through the load increases. Therefore, in order to perform reverse recovery within the dead time and minimize the reverse recovery loss, as the current flowing through the load increases, the reverse application is applied as in the second aspect of the power conversion circuit according to the present invention. It is desirable to increase the DC voltage.

  According to the air conditioner according to the present invention, an air conditioner with a small loss can be obtained.

  FIG. 1 is a circuit diagram showing a configuration of an inverter to which an embodiment of the present invention is applied and its periphery.

  Wirings 81 and 82 are provided as DC links. A positive DC voltage Ed is applied to the wiring 82 from the voltage source 1 with respect to the wiring 81. Here, a case where a capacitor is employed as the voltage source 1 is illustrated. For example, the capacitor is charged by a converter (not shown).

  Here, a three-phase load is assumed as the load 9, and specifically, for example, a three-phase motor is assumed. The load 9 has motor coils 91, 92, and 93 corresponding to each phase, and here, a case where these are star-connected (Y-connected) is illustrated. However, the present invention is effective even when these are circularly connected (Δ connection).

  Three legs are connected between the wirings 81 and 82. In the first leg, an IGBT 31 as a switching element on the high arm side and a MOSFET 41 as a switching element on the low arm side are connected in series. In the second leg, an IGBT 32 as a switching element on the high arm side and a MOSFET 42 as a switching element on the low arm side are connected in series. In the third leg, an IGBT 33 as a switching element on the high arm side and a MOSFET 43 as a switching element on the low arm side are connected in series.

  Here, the IGBTs 31, 32, and 33 are employed as the switching elements on the high arm side, but MOSFETs may be employed. In addition, each of the IGBTs 31, 32, and 33 is illustrated with a case where diodes 31b, 32b, and 33b having a forward direction opposite to the conduction direction of these IGBTs are illustrated, but these are provided separately. Instead of this, it may be provided incidentally to the IGBTs 31, 32, 33, or may be parasitic.

  Each of the MOSFETs 41, 42, and 43 has parasitic diodes 41a, 42a, and 43a having a forward direction from the source to the drain as a forward direction.

  A load 9 is connected to a connection point between the switching element on the high arm side and the switching element on the low arm side in each of the first to third legs. More specifically, the IGBT 31 is connected between the wiring 82 and the coil 91, and the MOSFET 41 is connected between the wiring 81 and the coil 91. Similarly, the IGBT 32 is connected between the wiring 82 and the coil 92, the MOSFET 42 is connected between the wiring 81 and the coil 92, the IGBT 33 is connected between the wiring 82 and the coil 93, and the wiring 81 is connected between the wiring 81 and the coil 93. The MOSFETs 43 are connected to each other.

  Hereinafter, reverse recovery in the parasitic diode 41a will be described including the embodiment of the present invention, but the same applies to reverse recovery in the parasitic diodes 42a and 43a. However, in order to avoid complication of the drawing, only the configuration for improving reverse recovery in the parasitic diode 41a is illustrated.

  In FIG. 1, a DC voltage source 52, a switching element 22, and a diode 23 are provided as a configuration for improving reverse recovery in the parasitic diode 41a.

  The DC voltage source 52 outputs a variable DC voltage Es that is smaller than the DC voltage Ed. The negative electrode is connected to the wiring 81, and the positive electrode is connected to one end of the switching element 22. The other end of the switching element 22 is connected to the anode of the diode 23, and the cathode of the diode 23 is connected to the drain of the MOSFET (and thus to the cathode of the parasitic diode 41a).

  The switching element 22 becomes conductive before the DC voltage Ed is applied in the reverse direction via the IGBT 31. Specifically, in the dead time when both the IGBT 31 and the MOSFET 41 are off, the parasitic diode 41a is reversely recovered using the DC voltage Es via the diode 23, and the parasitic diode 41a is reversely recovered using the DC voltage Ed. The loss at the time of reverse recovery is reduced as compared with the case of making it. The control of the switching element 22 can be easily realized by using a known technique. For example, a MOSFET can be used for the switching element 22.

  In addition, since the direct-current voltage Es that is the reverse voltage is variable, the reverse voltage is set to an appropriate value in order to complete reverse recovery in a so-called dead time according to the current flowing through the parasitic diode 41a and thus the coil 91. can do.

  The diode 23 is desirably provided in consideration of the case where the IGBT 31 and the switching element 22 are turned on during the dead time and the case where the MOSFET 41 and the switching element 22 are turned on. In such a case, a reverse current may flow through the switching element 22. In order to prevent the reverse current from flowing to the switching element 22 and destroying it, a function of blocking the reverse current by the diode 23 is desired. This is particularly suitable when a MOSFET having a low withstand voltage is employed as the switching element 22.

  FIG. 2 is a graph showing the current flowing through the parasitic diode 41a during reverse recovery. Graphs K1 and K3 show the case where the same forward voltage is applied before time t0, and the case where the reverse voltage is applied after time t0 (hereinafter referred to as Case I and Case III, respectively). . In case I, the reverse voltage is larger than in case III. The current flowing through the parasitic diode 41a decreases after the time t0, but becomes almost zero after transiently flowing in the reverse direction (the steady reverse current is ignored).

  The current flowing through the parasitic diode 41a becomes almost zero at times t1 and t3 in the graphs K1 and K3, respectively. In both graphs K1 and K3, the current decreases at time t0, and time t3 is later than time t1. Therefore, the period T1 (= t1-t0) required for reverse recovery in Case I is shorter than the period T3 (= t3-t0) required for reverse recovery in Case III.

  Therefore, in order to complete reverse recovery during the dead time, it is desirable that the reverse voltage is large. On the other hand, since the loss at the time of reverse recovery is calculated by the integral value of the current flowing in the reverse direction, it is desirable that the reverse voltage is small.

  Graph K2 shows a case where a forward current larger than that in cases I and III flows (hereinafter referred to as case II), and shows a case where a reverse voltage equal to case I is applied.

  The graphs K1 and K2 show a case where currents that coincide with each other after time t0 flow. Such a coincidence occurs when the time t2 when the reverse voltage is applied in case II is before the time t0 when the reverse voltage is applied in case I. In other words, the period T2 (= t1-t2) necessary for reverse recovery in Case II is longer than the period T (= t1-t0) required for reverse recovery in Case I. That is, if the reverse voltage is equal, the time required for reverse recovery increases as the forward current flowing through the parasitic diode 41a increases.

  When the current flowing through the coil 91 flows through the parasitic diode 41a, the forward current flowing through the parasitic diode 41a becomes the current flowing through the coil 91. Therefore, in order to perform reverse recovery within the dead time and minimize the reverse recovery loss, it is desirable to increase the DC voltage applied in the reverse direction as the current flowing through the load increases.

  FIG. 3 is a graph showing the relationship between the DC voltage Es applied for reverse recovery (reverse voltage) and the reverse recovery loss at that time, with the magnitude of the current I flowing through the coil 91 as a parameter. As can be seen from the comparison of the graphs K1 and K3 in FIG. 2, if the current I is the same, the reverse recovery loss increases as the DC voltage Es increases. Therefore, each graph in FIG. 3 is inclined upward.

  As can be seen from the comparison of the graphs K1 and K2 in FIG. 2, the reverse recovery loss increases as the current I increases, and the time required for reverse recovery increases as the reverse voltage of the graph is equal. Therefore, the respective graphs of FIG. 3 do not intersect, and the DC voltage Es for minimizing the reverse recovery loss while keeping the reverse recovery within the dead time increases as the current I increases. Therefore, it is desirable to set the DC voltage Es larger as the current I increases. As shown in FIG. 3, when the DC voltage Es set in this way is applied and the current I increases, the reverse recovery loss sequentially increases as indicated by the values L1, L2, and L3.

  Therefore, the voltage control circuit 51 detects the current I flowing from the connection point between the IGBT 31 and the MOSFET 41 to the coil 91, and controls the DC voltage of the DC voltage source 52 to be larger as the current I is larger.

  The voltage control circuit 51 and the DC voltage source 52 can be combined and understood as the variable voltage source 5.

  FIG. 4 is a block diagram illustrating the configuration of the air conditioner 8 including the inverter. The air conditioner 8 has an outdoor unit 82 and an indoor unit 83. The outdoor unit 82 includes a power conversion circuit 80, a load 9, and a compressor 81. The inverter is used as the power conversion circuit 80, and a motor is used as the load 9. The compressor is driven by the motor 9 and compresses the refrigerant. The refrigerant flows in and out of the indoor unit 83 via the path 84.

  In such an air conditioner 8, the reverse recovery loss is reduced as shown in the above-described operation, the loss of the power conversion circuit 80 is small, and the loss as a whole is also small.

1 is a circuit diagram showing a configuration of an inverter to which an embodiment of the present invention is applied and its periphery. It is a graph which shows the electric current which flows into a parasitic diode at the time of reverse recovery. It is a graph which shows the relationship between a reverse voltage and reverse recovery loss. It is a block diagram which illustrates the composition of the air harmony machine provided with the inverter with which the embodiment of the invention is applied.

Explanation of symbols

81, 82 Wiring 9 Load (motor)
91, 92, 93 Coil 22 Switching element 31, 32, 33 IGBT
41, 42, 43 MOSFET
41a, 42a, 43a Parasitic diode 52 DC voltage source 51 Voltage control circuit 81 Compressor 8 Air conditioner

Claims (3)

A first wiring (81);
A second wiring (82) to which a positive DC voltage (Ed) is applied to the first wiring;
A first switching element (31) connected between the first wiring and a load (91);
A MOS field-effect transistor (41) connected between the second wiring and the load (91) and having a parasitic diode (41a) that conducts current only in one direction from the second wiring to the load. )When,
A DC voltage source (52) for outputting a variable DC voltage;
A reverse voltage is applied to the parasitic diode only when the first switching element and the MOS field-effect transistor are both non-conductive and provided between the DC voltage source and the parasitic diode. A power conversion circuit comprising: a second switching element (22) for applying a voltage. The current (I) flowing out from the connection point between the first switching element and the MOS field effect transistor to the load is detected, and the DC voltage of the DC voltage source (52) is controlled to be larger as the current is larger. Voltage control circuit (51)
The power conversion circuit according to claim 1, further comprising: The power conversion circuit according to claim 1 or claim 2 is provided,
The load (91) is a coil provided in the motor (9),
An air conditioner (8) further comprising a compressor (81) driven by the motor (9) to compress the refrigerant.

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